Micro-machining makes it possible to produce accelerometers of very reduced size at relatively low cost since it is simultaneously possible to machine a large number of accelerometers on one and the same support slice before dividing them into individual chips.
However, even with this type of technology, it remains desirable to further reduce the dimensions and costs. Specifically, there are more and more applications in the mass-market sector (automobiles, mobile telephony, etc.) and in the professional sector (aeronautics, industrial robotics, etc.) where cost, on the one hand, and bulkiness, on the other hand, are major issues, the cost being moreover in general inversely proportional to size for a given technology.
In particular, in applications where one needs to detect accelerations in the three dimensions in space, three simple accelerometers were firstly used, each oriented in a different direction so as to measure the accelerations in each of these directions. This solution is cumbersome and expensive since it demands three micro-machined components and a mounting of these three components in different but very precise orientations on a common platform.
It has therefore been sought to produce triaxial accelerometers using a micro-machined common proof mass, provided with means for detecting the displacements of the mass along three independent axes.
In particular, triaxial accelerometers have been proposed in which the proof mass is relatively thick (100 to 200 micrometers for example) because it is sliced through the whole thickness of a silicon substrate. Detection is capacitive or piezoelectric or piezoresistive. This type of production involves fabrication by adhesive bonding of several silicon slices (or other materials): in general three slices; the technology is fairly expensive since it is necessary to envisage very deep etchings to define the proof mass, and these deep etchings then require significant surface areas, therefore corresponding bulkiness. Moreover, these sensors are often produced using substrate transfer techniques, thereby posing problems of dimensional inaccuracy in the direction perpendicular to the substrate, the response of these sensors to accelerations, that is to say the amplitude of the output signal as a function of acceleration, may then be inaccurate, not very linear. Moreover, the sensitivity axes are not very independent, that is to say an acceleration along one axis induces spurious signals on the outputs corresponding to the other axes. It is therefore necessary to envisage compensation systems, this also being a drawback.
To avoid these drawbacks, triaxial accelerometers whose proof mass is made not in the global thickness of a silicon substrate but in the much lower thickness of a layer suspended above a substrate have also been proposed. Detection is capacitive, with interdigitated combs for the horizontal axes, and with plane capacitances (between the proof mass and the substrate situated below the mass). If it is necessary to slave the position of the mass to a central rest position, as is often desirable to improve performance, it is possible to do so along the horizontal axes, by using interdigitated combs assigned to this slaving, but it is not possible to do so in both directions (up and down) of the vertical axis without considerably complicating the technology.
The article “A 3-Axis Force Balanced Accelerometer Using a single Proof-Mass”, by Mark A. Lemkin, Bernhard E. Boser, David Auslander, Jim H. Smith, published in the IEEE Transducers '97 proceedings, following the conference in 1997 “International Conference on Solid-State Sensors and Actuators”, describes such an accelerometer with surface technology machined proof mass.